Question

1.     Tính nhanh: 1/1x2 + 1/2x3 + 1/3x4 + …+ 1/1981x1982

Answer 1

=1-1/2+1/2-1/3+...+1/1981-1/1982

=1-1/1982

=1981/1982

Answer 2

Lời giải:

$\frac{1}{1\times 2}+\frac{1}{2\times 3}+\frac{1}{3\times 4}+....+\frac{1}{1981\times 1982}$

$=\frac{2-1}{1\times 2}+\frac{3-2}{2\times 3}+\frac{4-3}{3\times 4}+...+\frac{1982-1981}{1981\times 1982}$

$=1-\frac{1}{2}+\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+...+\frac{1}{1981}-\frac{1}{1982}$

$=1-\frac{1}{1982}=\frac{1981}{1982}$

Answer 3

=1-1/2+1/2-1/3+...+1/1981-1/1982

=1-1/1982

=1981/1982